Because I was tyig to squeeze the ube with 1 hand only (because there was only one plastic glove in the pack - which I now realise was probably to keep me safe when collecting the water from the river and not for stopping my hands from contaminating the experiment) and I hadn't realised that you really need to squeeze the tube on the FLAT sides (not the edges - which is what I'd understood from the instruction to "press the sides of the tube") I hadn't got the full 1.5ml of water in the tube for the Nitrogen test. I don't know if ths affected the result. I was surprised that it was so high because I thought the water quality in this part of the non-tidal Thames is good.
I'm afraid I thought the paper instructions were unclear - thank goodness I watched the video. The instructions didn't explain that you should collect your sample in a separate receptacle, or that there are 2 tubes and they are marked NO3 and PO4 in tiny writing and you need to use the right one for the right test (initialy, I thought maybe 1 tube was a spare and you simply compared the colours on the N card after minutes and then the same tube on the P card after 2 further minutes - I only worked it out after watching the video).